This invention relates generally to the measuring of the volume of liquid in a tank using ultrasound, and, in particular, relates to the measuring of the volume of liquid in underground fuel storage tanks.
Many liquids are stored in tanks where the volume of the liquid in the tank cannot be directly observed. In many instances, the liquids are fuels such as gasoline and the tanks are large underground tanks. In the past a very common way of determining the volume in such a tank was to insert a calibrated rod and read the height of the liquid from the line formed on the rod by the liquid surface. This method, however, is not very precise and there are inherent errors in utilizing such an approach. One very serious drawback of the lack of precision is the inability to detect the loss of gasoline due to leakage. It is readily apparent that a leakage of gasoline or fuel from underground tanks can cause substantial environmental problems. It is desirable to detect such leaks before the problems become severe. Furthermore, state and federal regulations are beginning to mandate requirements for the detection of fuel leakage.
Currently, the EPA requires underground storage tanks to undergo leak tests. The EPA requires the tanks to be checked monthly with a system capable of detecting a 0.2 gallons per hour leak or yearly with a system capable of detecting a 0.1 gallons per hour leak. Systems presently on the market generally measure depth changes of less than 0.001 inches and temperature changes of less than 0.001.degree. C.
The market is extremely price sensitive because tank owners desire to comply with regulations at minimal expense. Liquid measuring systems which utilize magnetostrictive and ultrasonic probes are the most popular in the market today. Magnetostrictive probes are higher priced giving ultrasonic probes a price advantage. Thus, it is desirable to provide an ultrasonic system.
Systems are now available which utilize ultrasonic transducers and ultrasound to determine the volume of liquid and typically place the transducer at the end of a probe and insert the probe into the fuel tank. The probe typically includes reflectors spaced along its length at precisely known distances from the transducer.
In one such known arrangement, the reflectors appear on the outside surface of the probe and are equally spaced apart. In operation, such a prior art system periodically transmits bursts of ultrasonic energy from the transducer. The burst of sonic energy produces echoes off of each of the submerged reflectors and off of the liquid/air surface interface. The echo received from the surface is typically the strongest echo because the large surface area reflects a greater amount of energy than would any of the reflectors positioned on the probe. The transmitted burst typically includes several ringing cycles. The echo signal waveforms likewise includes several ringing cycles. In the prior system, a receiver sensitivity is initially set at a minimum value and is successively increased until an echo signal is received. From that point, a series of echoes are analyzed in order to identify the first cycle in the echo signal waveform. Once the first cycle has been identified, it is used as a reference point to determine the echo delay times. The system then proceeds through an algorithm to identify every submerged reflector with the goal of identifying the topmost submerged reflector. When the topmost submerged reflector is identified, the echo delay time from the topmost reflector is utilized to calculate an average sonic velocity through the fuel. Based upon this average sonic velocity and the echo time received from the surface of the fuel, the depth of the liquid in the tank is calculated.
In addition, many approaches have been utilized to compensate for variations in the volume of the liquid with temperature. Again, in the prior above identified system, a temperature reading is taken at a point near the ultrasonic transducer. The sonic velocity between the transducer and the lowest reflector is measured to produce a reference sonic velocity at a reference temperature. The prior systems then proceed to determine an average temperature directly from the average sonic velocity in the liquid. The calculated volume of liquid in the tank is then adjusted to compensate for the temperature which is determined from the change in the average sonic velocity.
Still a further desirable aspect of any such measuring or gauging system is to accurately determine the depths of water in the fuel tank. In the prior system, the transducer is suspended in the tank such that it would be above the fuel/water interface. By relying upon multiple reflection of the ultrasonic energy from the transducer to the fuel/air interface back to the water/fuel interface and reflected back to the fuel/air interface and back to the transducer, the level of the water in the tank would be calculated.
The present invention is directed to providing an improved ultrasonic apparatus for measuring and remotely displaying the amount of liquid in a tank that provides for significantly improved accuracy in determining the volume and significantly improved accuracy in determining a temperature compensated volume and further, more accurately measuring the volume of water in a tank so that the volume of fuel can be more accurately calculated.